The invention relates to in vivo dosimetry techniques that are used to record the dose received by patient during radiation therapy treatment and for detection of errors during treatment delivery. It can be applied for comparison of the dose measured in reference point and dose calculated at the same place with computerized treatment planning system (TPS) such as Pinnacle or Eclipse.
The measurement of a total dose that a patient receives during a radiotherapy process is usually implemented with a small dosimeter like diode, TLD or OSLD (nanodot) which is placed on a patient's skin at the location prescribed by a physician or selected by a dosimetrist. The dosimeter is often covered by bolus, a tissue equivalent material with about 0.5 cm thickness or more depending on the energy of applied radiation (report of TG 62 of the Radiation Therapy Committee “Diode in vivo dosimetry for patients receiving external beam radiation therapy,” AAPM report #87, Medical Physics Publishing, Madison, Wis., 2005; International Atomic Energy, “Development of procedures for in vivo dosimetry in radiotherapy” “IAEA Human Health Report #8, Vienna, Austria, 2013; European Society of Radiation Oncology, “Practical guidelines for implementation of in vivo dosimetry with diodes in external radiotherapy with photon beams (entrance dose),” ESTRO Booklet #5, Brussels, Belgium, 2001).
A dosimeter can also be placed in a patient hollow organ or be temporary implanted in tissue. Then a patient is irradiated by a beam of X-rays, electrons or other ionizing radiation. After the procedure is finished the dose received by dosimeter is measured with a special device depending on the type of dosimeter. Then the measured dose is compared with the dose at the point of measurements (POM) calculated with TPS.
At present computerized TPS uses points of interest (POI) or reference points to define the dose in the POM. The dose is calculated in the center of the most probable dosimeter location and in several points around it to provide for uncertainties of dosimeter setup and patient's movement. The average dose is defined as the calculated dose in the point of measurement during the radiotherapy procedure. The main shortcoming of this method is that POI are selected arbitrary and a measured dose at the POM can significantly differ from the calculated one (Mijnheer et al. “In vivo dosimetry in external beam radiotherapy,” Med. Phys., 40, July 2013). The difference is especially large in brachytherapy dosimetry due to high gradient dose distribution and the large range of dose and dose rate (Tanderum et al. “In vivo dosimetry in Brachytherapy). This discrepancy and impossibility to estimate a confidence interval can cause wrong interpretation of the results of comparison.
Electronic Portal imaging devises are also used for in vivo dosimetry which can provide two- and three-dimensional dosimetric information delivered to patient (W. van Elmpt et al, “3D in vivo dosimetry using megavoltage cone-beam CT and EPID dosimetry,” Int. J. Radiat. Oncol., Biol., Phys. 731580-1587, 2009). However, they are oversensitive to photons of low energy, have continued signal after irradiation ceased, and the accuracy of measurements is often not enough because of influence of scattered radiation. Sometimes special devises and software are used for 2D or 3D gamma analysis and DVH comparison during pretreatment IMRT and VMAT verification of dose distribution (Carrasco et al. “3D DVH-based metric analysis versus per-beam planar analysis in IMRT pretreatment verification,” Med. Phys. 39, 5040-5049, 2012). However, at this time no pass/fail criteria are available for differences of measured and planned doses. Besides, these techniques use special equipment and sophisticated software, have essentially the same disadvantages as EPIDs, applied for the measurements of dose distribution rather that absolute dose evaluation in the point or region of interest, work with the dose distribution registered for the a certain moment of time and are not practical for routine in vivo dose measurements.